1 Introduction

This document lists C++ coding recommendations common in the C++ development community.

The recommendations are based on established standards collected from a number of sources, individual experience, local requirements/needs, as well as suggestions given in [1] - [4].

There are several reasons for introducing a new guideline rather than just referring to the ones above. The main reason is that these guides are far too general in their scope and that more specific rules (especially naming rules) need to be established. Also, the present guide has an annotated form that makes it far easier to use during project code reviews than most other existing guidelines. In addition, programming recommendations generally tend to mix style issues with language technical issues in a somewhat confusing manner. The present document does not contain any C++ technical recommendations at all, but focuses mainly on programming style. For guidelines on C++ programming style refer to the C++ Programming Practice Guidelines.

While a given development environment (IDE) can improve the readability of code by access visibility, color coding, automatic formatting and so on, the programmer should never rely on such features. Source code should always be considered larger than the IDE it is developed within and should be written in a way that maximise its readability independent of any IDE.

1.1 Layout of the Recommendations.

The recommendations are grouped by topic and each recommendation is numbered to make it easier to refer to during reviews.

Layout of the recommendations is as follows:

n. Guideline short description

Example if applicable

Motivation, background and additional information.

The motivation section is important. Coding standards and guidelines tend to start "religious wars", and it is important to state the background for the recommendation.

1.2 Recommendation Importance

In the guideline sections the terms must, should and can have special meaning. A must requirement must be followed, a should is a strong recommendation, and a can is a general guideline.

2 General Recommendations

1. Any violation to the guide is allowed if it enhances readability.

The main goal of the recommendation is to improve readability and thereby the understanding and the maintainability and general quality of the code. It is impossible to cover all the specific cases in a general guide and the programmer should be flexible.

2. The rules can be violated if there are strong personal objections against them.

The attempt is to make a guideline, not to force a particular coding style onto individuals. Experienced programmers normally want to adopt a style like this anyway, but having one, and at least requiring everyone to get familiar with it, usually makes people start thinking about programming style and evaluate their own habits in this area.

On the other hand, new and inexperienced programmers normally use a style guide as a convenience of getting into the programming jargon more easily.

3 Naming Conventions

3.1 General Naming Conventions

3. Names representing types must be in mixed case starting with upper case.

Line, SavingsAccount

Common practice in the C++ development community.

4. Variable names must be in mixed case starting with lower case.

line, savingsAccount

Common practice in the C++ development community. Makes variables easy to distinguish from types, and effectively resolves potential naming collision as in the declaration Line line;

5. Named constants (including enumeration values) must be all uppercase using underscore to separate words.

MAX_ITERATIONS, COLOR_RED, PI

Common practice in the C++ development community. In general, the use of such constants should be minimized. In many cases implementing the value as a method is a better choice:

int getMaxIterations() // NOT: MAX_ITERATIONS = 25
{
return 25;
}

This form is both easier to read, and it ensures a unified interface towards class values.

6. Names representing methods or functions must be verbs and written in mixed case starting with lower case.

getName(), computeTotalWidth()

Common practice in the C++ development community. This is identical to variable names, but functions in C++ are already distingushable from variables by their specific form.

7. Names representing namespaces should be all lowercase.

model::analyzer, io::iomanager, common::math::geometry

Common practice in the C++ development community.

8. Names representing template types should be a single uppercase letter.

template<class T> ...
template<class C, class D> ...

Common practice in the C++ development community. This makes template names stand out relative to all other names used.

9. Abbreviations and acronyms must not be uppercase when used as name [4].

Using all uppercase for the base name will give conflicts with the naming conventions given above. A variable of this type whould have to be named dVD, hTML etc. which obviously is not very readable. Another problem is illustrated in the examples above; When the name is connected to another, the readbility is seriously reduced; the word following the abbreviation does not stand out as it should.

10. Global variables should always be referred to using the :: operator.

::mainWindow.open(), ::applicationContext.getName()

In general, the use of global variables should be avoided. Consider using singleton objects instead.

11. Private class variables should have underscore suffix.

class SomeClass {
private:
int length_;
}

Apart from its name and its type, the scope of a variable is its most important feature. Indicating class scope by using underscore makes it easy to distinguish class variables from local scratch variables. This is important because class variables are considered to have higher significance than method variables, and should be treated with special care by the programmer.

A side effect of the underscore naming convention is that it nicely resolves the problem of finding reasonable variable names for setter methods and constructors:

void setDepth (int depth)
{
depth_ = depth;
}

An issue is whether the underscore should be added as a prefix or as a suffix. Both practices are commonly used, but the latter is recommended because it seem to best preserve the readability of the name.

It should be noted that scope identification in variables has been a controversial issue for quite some time. It seems, though, that this practice now is gaining acceptance and that it is becoming more and more common as a convention in the professional development community.

Reduce complexity by reducing the number of terms and names used. Also makes it easy to deduce the type given a variable name only.

If for some reason this convention doesn't seem to fit it is a strong indication that the type name is badly chosen.

Non-generic variables have a role. These variables can often be named by combining role and type:

Point startingPoint, centerPoint;
Name loginName;

13. All names should be written in English.

fileName; // NOT: filNavn

English is the preferred language for international development.

14. Variables with a large scope should have long names, variables with a small scope can have short names [1].

Scratch variables used for temporary storage or indices are best kept short. A programmer reading such variables should be able to assume that its value is not used outside of a few lines of code. Common scratch variables for integers are i, j, k, m, n and for characters c and d.

15. The name of the object is implicit, and should be avoided in a method name.

line.getLength(); // NOT: line.getLineLength();

The latter seems natural in the class declaration, but proves superfluous in use, as shown in the example.

3.2 Specific Naming Conventions

17. The terms get/set must be used where an attribute is accessed directly.

There are two types of words to consider. First are the common words listed in a language dictionary. These must never be abbreviated. Never write:

cmd instead of commandcp instead of copypt instead of pointcomp instead of computeinit instead of initializeetc.

Then there are domain specific phrases that are more naturally known through their abbreviations/acronym. These phrases should be kept abbreviated. Never write:

HypertextMarkupLanguage instead of htmlCentralProcessingUnit instead of cpuPriceEarningRatio instead of peetc.

29. Naming pointers specifically should be avoided.

Line* line; // NOT: Line* pLine;
// NOT: LIne* linePtr;

Many variables in a C/C++ environment are pointers, so a convention like this is almost impossible to follow. Also objects in C++ are often oblique types where the specific implementation should be ignored by the programmer. Only when the actual type of an object is of special significance, the name should emphasize the type.

30. Negated boolean variable names must be avoided.

bool isError; // NOT: isNoError
bool isFound; // NOT: isNotFound

The problem arises when such a name is used in conjunction with the logical negation operator as this results in a double negative. It is not immediately apparent what !isNotFound means.

31. Enumeration constants can be prefixed by a common type name.

enum Color {
COLOR_RED,
COLOR_GREEN,
COLOR_BLUE
};

This gives additional information of where the declaration can be found, which constants belongs together, and what concept the constants represent.

An alternative approach is to always refer to the constants through their common type: Color::RED, Airline::AIR_FRANCE etc.

Note also that the enum name typically should be singular as in enum Color {...}. A plural name like enum Colors {...} may look fine when declaring the type, but it will look silly in use.

32. Exception classes should be suffixed with Exception.

class AccessException
{
:
}

Exception classes are really not part of the main design of the program, and naming them like this makes them stand out relative to the other classes.

33. Functions (methods returning something) should be named after what they return and procedures (void methods) after what they do.

Increase readability. Makes it clear what the unit should do and especially all the things it is not supposed to do. This again makes it easier to keep the code clean of side effects.

4 Files

4.1 Source Files

34. C++ header files should have the extension .h (preferred) or .hpp. Source files can have the extension .c++ (recommended), .C, .cc or .cpp.

MyClass.c++, MyClass.h

These are all accepted C++ standards for file extension.

35. A class should be declared in a header file and defined in a source file where the name of the files match the name of the class.

MyClass.h, MyClass.c++

Makes it easy to find the associated files of a given class. An obvious exception is template classes that must be both declared and defined inside a .h file.

The header files should declare an interface, the source file should implement it. When looking for an implementation, the programmer should always know that it is found in the source file.

37. File content must be kept within 80 columns.

80 columns is a common dimension for editors, terminal emulators, printers and debuggers, and files that are shared between several people should keep within these constraints. It improves readability when unintentional line breaks are avoided when passing a file between programmers.

38. Special characters like TAB and page break must be avoided.

These characters are bound to cause problem for editors, printers, terminal emulators or debuggers when used in a multi-programmer, multi-platform environment.

By this, the programmer indicates that he is aware of the different types involved and that the mix is intentional.

5.2 Variables

46. Variables should be initialized where they are declared.

This ensures that variables are valid at any time. Sometimes it is impossible to initialize a variable to a valid value where it is declared:

int x, y, z;
getCenter(&x, &y, &z);

In these cases it should be left uninitialized rather than initialized to some phony value.

47. Variables must never have dual meaning.

Enhance readability by ensuring all concepts are represented uniquely. Reduce chance of error by side effects.

48. Use of global variables should be minimized.

In C++ there is no reason global variables need to be used at all. The same is true for global functions or file scope (static) variables.

49. Class variables should never be declared public.

The concept of C++ information hiding and encapsulation is violated by public variables. Use private variables and access functions instead. One exception to this rule is when the class is essentially a data structure, with no behavior (equivalent to a C struct). In this case it is appropriate to make the class' instance variables public [2].

Note that structs are kept in C++ for compatibility with C only, and avoiding them increases the readability of the code by reducing the number of constructs used. Use a class instead.

51. C++ pointers and references should have their reference symbol next to the type rather than to the name.

float* x; // NOT: float *x;
int& y; // NOT: int &y;

The pointer-ness or reference-ness of a variable is a property of the type rather than the name. C-programmers often use the alternative approach, while in C++ it has become more common to follow this recommendation.

53. Implicit test for 0 should not be used other than for boolean variables and pointers.

do-while loops are less readable than ordinary while loops and for loops since the conditional is at the bottom of the loop. The reader must scan the entire loop in order to understand the scope of the loop.

In addition, do-while loops are not needed. Any do-while loop can easily be rewritten into a while loop or a for loop. Reducing the number of constructs used enhance readbility.

58. The use of break and continue in loops should be avoided.

These statements should only be used if they give higher readability than their structured counterparts.

60. The form while(true) should be used for infinite loops.

while (true) {
:
}
for (;;) { // NO!
:
}
while (1) { // NO!
:
}

Testing against 1 is neither necessary nor meaningful. The form for (;;) is not very readable, and it is not apparent that this actually is an infinite loop.

Conditionals with executable statements are just very difficult to read. This is especially true for programmers new to C/C++.

5.5 Miscellaneous

65. The use of magic numbers in the code should be avoided. Numbers other than 0 and 1 should be considered declared as named constants instead.

If the number does not have an obvious meaning by itself, the readability is enhanced by introducing a named constant instead. A different approach is to introduce a method from which the constant can be accessed.

66. Floating point constants should always be written with decimal point and at least one decimal.

This emphasizes the different nature of integer and floating point numbers. Mathematically the two model completely different and non-compatible concepts.

Also, as in the last example above, it emphasizes the type of the assigned variable (sum) at a point in the code where this might not be evident.

67. Floating point constants should always be written with a digit before the decimal point.

double total = 0.5; // NOT: double total = .5;

The number and expression system in C++ is borrowed from mathematics and one should adhere to mathematical conventions for syntax wherever possible. Also, 0.5 is a lot more readable than .5; There is no way it can be mixed with the integer 5.

68. Functions must always have the return value explicitly listed.

int getValue() // NOT: getValue()
{
:
}

If not exlicitly listed, C++ implies int return value for functions. A programmer must never rely on this feature, since this might be confusing for programmers not aware of this artifact.

69. goto should not be used.

Goto statements violate the idea of structured code. Only in some very few cases (for instance breaking out of deeply nested structures) should goto be considered, and only if the alternative structured counterpart is proven to be less readable.

70. "0" should be used instead of "NULL".

NULL is part of the standard C library, but is made obsolete in C++.

6 Layout and Comments

6.1 Layout

71. Basic indentation should be 2.

for (i = 0; i < nElements; i++)
a[i] = 0;

Indentation of 1 is too small to emphasize the logical layout of the code. Indentation larger than 4 makes deeply nested code difficult to read and increases the chance that the lines must be split. Choosing between indentation of 2, 3 and 4, 2 and 4 are the more common, and 2 chosen to reduce the chance of splitting code lines.

72. Block layout should be as illustrated in example 1 below (recommended) or example 2, and must not be as shown in example 3 [4]. Function and class blocks must use the block layout of example 2.

while (!done) {
doSomething();
done = moreToDo();
}

while (!done)
{
doSomething();
done = moreToDo();
}

while (!done)
{
doSomething();
done = moreToDo();
}

Example 3 introduces an extra indentation level which doesn't emphasize the logical structure of the code as clearly as examples 1 and 2.

This follows partly from the general block rule above. However, it might be discussed if an else clause should be on the same line as the closing bracket of the previous if or else clause:

if (condition) {
statements;
} else {
statements;
}

The chosen approach is considered better in the way that each part of the if-else statement is written on separate lines of the file. This should make it easier to manipulate the statement, for instance when movingelse clauses around.

76. A for statement should have the following form:

for (initialization; condition; update) {
statements;
}

This follows from the general block rule above.

77. An empty for statement should have the following form:

for (initialization; condition; update)
;

This emphasizes the fact that the for statement is empty and it makes it obvious for the reader that this is intentional. Empty loops should be avoided however.

Note that each case keyword is indented relative to the switch statement as a whole. This makes the entire switch statement stand out. Note also the extra space before the : character. The explicit Fallthroughcomment should be included whenever there is a case statement without a break statement. Leaving the break out is a common error, and it must be made clear that it is intentional when it is not there.

81. A try-catch statement should have the following form:

try {
statements;
}
catch (Exception& exception) {
statements;
}

This follows partly from the general block rule above. The discussion about closing brackets for if-else statements apply to the try-catch statments.

82. Single statement if-else, for or while statements can be written without brackets.

It is a common recommendation that brackets should always be used in all these cases. However, brackets are in general a language construct that groups several statements. Brackets are per definition superfluous on a single statement. A common argument against this syntax is that the code will break if an additional statement is added without also adding the brackets. In general however, code should never be written to accommodate for changes that might arise.

83. The function return type can be put in the left column immediately above the function name.

void
MyClass::myMethod(void)
{
:
}

This makes it easier to spot function names within a file since they all start in the first column.

6.2 White Space

84.- Conventional operators should be surrounded by a space character. - C++ reserved words should be followed by a white space. - Commas should be followed by a white space. - Colons should be surrounded by white space. - Semicolons in for statments should be followed by a space character.

Makes the individual components of the statements stand out. Enhances readability. It is difficult to give a complete list of the suggested use of whitespace in C++ code. The examples above however should give a general idea of the intentions.

85. Method names can be followed by a white space when it is followed by another name.

doSomething (currentFile);

Makes the individual names stand out. Enhances readability. When no name follows, the space can be omitted (doSomething()) since there is no doubt about the name in this case.

An alternative to this approach is to require a space after the opening parenthesis. Those that adhere to this standard usually also leave a space before the closing parentheses: doSomething( currentFile );. This do make the individual names stand out as is the intention, but the space before the closing parenthesis is rather artificial, and without this space the statement looks rather asymmetrical (doSomething( currentFile);).

86. Logical units within a block should be separated by one blank line.

There are a number of places in the code where white space can be included to enhance readability even if this violates common guidelines. Many of these cases have to do with code alignment. General guidelines on code alignment are difficult to give, but the examples above should give a general clue.

6.3 Comments

90. Tricky code should not be commented but rewritten! [1]

In general, the use of comments should be minimized by making the code self-documenting by appropriate name choices and an explicit logical structure.

91. All comments should be written in English [2].

In an international environment English is the preferred language.

92. Use // for all comments, including multi-line comments.

// Comment spanning
// more than one line.

Since multilevel C-commenting is not supported, using // comments ensure that it is always possible to comment out entire sections of a file using /* */ for debugging purposes etc.

There should be a space between the "//" and the actual comment, and comments should always start with an upper case letter and end with a period.

93. Comments should be included relative to their position in the code. [1]

Regarding standardized class and method documentation the Java development community is more mature than the C/C++ one. This is due to the standard automatic Javadoc tool that is part of the development kit and that help producing high quality hypertext documentation from these comments.

There are Javadoc-like tools available also for C++. These follows the same tagging syntax as Javadoc. See for instance Doc++ or Doxygen.